Journal of Ovonic Research Vol. 14, No. 5, September – October 2018, p. 371 - 379

THERMAL PROPERTIES OF PARAFFIN/ NANO-MAGNETITE-

TREVORITE PHASE CHANGE MATERIALS

F. R. SAEED, M. H. A. A. AL-TIMIMI, W. H. A. AL-BANDA,

M. Z. ABDULLAH, I. STAMATIN, S. VOINEA, B. DOBRICA, A. E. BALAN*

University of Bucharest, Faculty of Physics, 3Nano-SAE Research Centre, 405

Atomistilor str., PO Box MG-38, Bucharest-Măgurele, Romania

Storage of latent heat using organic phase change nano-composite materials based on

paraffin-nanoparticles shows a growing interest in thermal energy storage systems.

Paraffin low thermal conductivity is the main drawback associated with such applications.

To tackle this issue, a composite material based on paraffin with magnetite-trevorite

nanoparticles dispersed in different ratios (0.5-2%) in paraffin has been studied. The

nanoparticles were synthesized using co-precipitation method with microwave heat

treatment and their structure and morphology were investigated by X-ray diffraction and

scanning electron microscopy. Differential scanning calorimetry was used to identify the

thermal properties of the composite materials. A special test rig was designed to

investigate the effect of nanoparticles addition on the heat transfer rate as well as melting

front velocity and charging time. Results show an acceptable thermal behavior of the

composite, which makes it a candidate for heat transfer improvement in thermal energy

storage applications.

(Received June 18, 2018; Accepted October 4, 2018)

Keywords: Phase change material, Nanoparticles, Thermal energy storage

1. Introduction

Phase change materials (PCMs) attracted considerable attention as a part of thermal

energy storage applications. PCMs have the ability to absorb and release thermal energy during

phase changing at almost constant temperature. Storage of energy through latent heat is the main

motive behind applying of PCMs such as paraffin in many applications like solar energy, building

energy saving, and thermal management systems of electronic devices. However, low thermal

conductivity associated with using of organic PCMs can consider a vital obstacle in the way of

using organic PCMs. Fins [1-2], metal foam [3], and fine particles are the most used techniques for

thermal conductivity enhancement.

Particularly, nanoparticle addition has drawn attention to researchers over the last years. A

nano-copper particle dispersed in water inside a square storage model was presented as one of the

first numerical reports on nano-fluid solidification [4]. The evolution of solid/liquid interface was

detected for pure PCM and for PCM/nano-copper, showing no effective increase in speed of the

melting front phase interface at earlier stages of solidification due to nanoparticles addition.

However, the impact of nano-copper particles was more effective at the melting interface with

higher mass fractions. Numerical results showed that the melting rate of paraffin was enhanced

after adding nanoparticles, i.e. adding 1 wt% Cu nanoparticles saves 13.1% of the melting time of

paraffin, in agreement with previous researches [5].

An experimental and numerical study on melting of n-octadecane/CuO nanoparticles was

performed in a square enclosure supplied to constant heat flux from one side and thermally

insulated from the other sides [6]. Finite element method (FEM) was used to solve the coupled

continuity, momentum, and energy equations in the numerical part. Both of experimental and

numerical results evidence an improvement in thermal conductivity as a result of CuO

nanoparticles effect, which leads to an increase of the heat transfer rate, in other words minimizing

*Corresponding author: andronie@3nanosae.org

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the charging time. Also, they have discussed the agglomeration and precipitation impact at high

loadings of nanoparticles due to viscosity increase.

Mesoporous silica dispersed in n-octadecane as PCM for thermal energy storage (TES)

applications was achieved [7]. Results indicate that the thermal conductivity has been improved,

for both liquid/solid phase in range of temperatures between 5-55 ˚C. On the other side, the

viscosity increased with 60% relative to simple PCM for 5wt% loading at 35 ˚C.

Carbon nanomaterials were also used to enhance the thermal conductivity of phase change

material, such as single walled carbon nanotube (SWCNT) [8], multi walled carbon nanotube

(MWCNT) [9] and graphene [10]. Results showed that specific heat increased with increasing

specific surface area of nano-materials, which can enhance the specific heat with 18.57% of

carbonate salt (CPCM). Thermal conductivity of CPCM promoted to the 56.98% with addition of

columnar structure of nano-materials such as SWCNT. All nanoparticle additives have limited

effect on the total amount of thermal TES capacity [11]. Nanoparticle constriction (i.e. the contact

area between linked nanoparticles) was also studied. Effective thermal conductivity of different

MWCNT/ paraffin nano-composites was measured. The rate of heat transfer between the

contacting MWCNTs can be increased by the factor 27, with diameter around 58nm [12].

The effect of nano-magnetite (Fe3O4) on thermal behavior of paraffin wax as PCM has

been studied. Synthesis of nano-magnetite had been achieved by sol-gel method. They indicate an

improvement in thermal conductivity by 48% and 67% for 10% and 20%, respectively. A slight

improvement in latent heat capacity was also recorded, while melting temperature of the mixture

was stable [13]. Moreover, another study on Fe3O4 nanoparticle enhanced paraffin wax showed

that nanoparticle addition induces modifications in the activation energy of the melting process

[14]. In the present work, authors propose a new composite: paraffin with magnetite-trevorite

obtained by a simplified co-precipitation method, combined with microwave heat treatment. Nano-

particles structure affected directly by thermal treatment during the synthesis. Microwave heat

treatment can play essential role on the shape [15], size [16], and evolution of the synthesis

process [17]. The aim of this work is investigate the effect of magnetite-trevorite particles on

thermal behavior of paraffin. Different nanoparticle loadings were added into paraffin wax by

mixing and melting processes. Thermal properties of the composites were measured

experimentally in terms of latent heat and heat capacity and heat transfer rate.

2. Experimental 2.1 Materials

Materials used in this research work were paraffin (RT58, Sigma Aldrich), ferrous

chloride (FeCl2.4H2O Merck, 98%), nickel chloride (NiCl2.6H2O, Merck, 99.9%), ethylene glycol,

Merck, 99.8%), sodium hydroxide (NaOH, pellets for analysis, Merck), hydrazine (N2H4.H2O,

Merck, 64-65 %, reagent grade, 98%), de-ionized distilled water was used in the preparation of all

the solutions.

2.2 Synthesis of magnetite / trevorite compound

Nanoparticles synthesis was achieved by co-precipitation method based on chemical

reduction of metals chlorides. 3.2g of FeCl2.4H2O and 0.8g of NiCl2.6H2O at molar ratio Fe/Ni 5/1

were dissolved in 100 ml of ethylene glycol of (99%). Afterwards, a mixer of high ultra-sonic

frequency was used to disperse the metals chlorides in ethylene glycol. The reducing agent- 2M

NaOH solution- is added dropwise to set the pH between 10 and 10.5 during stirring at 6000rpm.

The resulting solution is then subjected to microwave heat treatment at 140 ˚C for 40 minutes. The

pH of the solution is adjusted to 10.5-11 by adding hydrazine. Finally, microwave heat treatment

was applied again to the solution for 2 hours at 100 ˚C that results in black powder precipitation of

magnetite/trevorite nanoparticles. The precipitate particles are washed with distillated water

repeatedly until pH 7 and dried in an air oven for 24 hour at 50˚C.

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2.3 Preparation of Paraffin / Nano M-T nanoparticles mixture

Paraffin / magnetite-trevorite composite was prepared by dispersion and melting

procedures. Paraffin is melted at 70oC for 50 minutes and, then, magnetite/trevorite particles are

mixed into the paraffin using an ultrasonic bath for 2 hours. Three different loadings of

nanoparticles: 0.5, 1, and 2 wt% are added to the liquid paraffin to form the composite.

2.4 Characterization methods

The synthesized nanoparticles were characterized by XRD analysis of Panalytical X'pert

PRO MPD equipment, with the following operation conditions, the 2theta interval: 35-120

deg, step size 0.02 grade, time/step = 2 sec, 40 mA current and 45 kV voltage. The morphology

and microstructure of the nanoparticles are characterized using Scanning Electron Microscopy

(SEM, JIB-4600F), with 20 kV of tension acceleration. Thermal analysis are performed by means

of differential scanning calorimeter (DSC Mettler Toledo, model Star1) in nitrogen atmosphere at

10oC/min heating rate, in the temperature range 20-90

oC. The 5-10 mg samples are sealed in

aluminium pans. Indium was used as a calibration reference of DSC. Each sample was treated for

30 min at 90oC to lose the thermal memory effect. The heat transfer rate during melting and the

effect of nanoparticles adding to the paraffin on thermal conductivity properties were investigated

through an in-house test rig. The schematic representation of experimental setup of rectangular

container is shown in Fig. 1. The test rig enclosure is made of fiberglass plates with internal

dimensions of 100 × 25 × 20 mm. The thermal conductivity of the fiberglass is about 0. 4 W/m.K,

which can consider its low enough to reduce heat losses from the test rig.

Fig. 1. Schematic diagram for the experimental setup.

Table 1. (x, y) coordinates of the thermocouples in mm for the test rig.

TC1 (1,9)

TC3 (33,18) TC5 (66,18) TC7 (99,18)

TC2 (33,8) TC4 (66,8) TC6 (99,8)

An aluminum plate of high thermal conductivity (218 W/m.K) is fixed on the right side of

the rectangular enclosure to obtain uniform heat flux applied on the plate by an electric heater. The

0.254 mm- thick electric heater has a maximum heat flux of about 1.55x104 W/m

2 at 120 V. This

heater is firmly attached to the outer surface of the aluminum plate. A power supply is applied to

perform the constant heat rate as the voltage can be changed while the electric current changes.

Seven K-type thermocouples (TCs) with 0.26 mm bare wire diameter were used for monitoring the

temperature of the sample inside the container. TCs were positioned at different locations by using

clear epoxy (see Table 1). 8-channel data acquisition (PICO) unit was used to record the TCs

readings. The Data Logger unit was connected to a computer and operated through a data recorder

program. Prior measurements, TCs were calibrated.

3. Results and discussions

3.1 XRD

Debye-Scherrer formula was used to calculate the average crystalline size of the

synthesized nano-particles which was around 10 nm. The identification of the peaks was carried

out using data base ICCD (International Center for Diffraction date) with the pdf card for the

nickel-Iron-oxide 04-014-8285. Calculating the size of crystallization by considering Nickel-Iron

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oxide diffraction peaks (2θ = 35.68˚), corresponding to Miller indices (311). This is revealed that

the resultant particles were face-centered cubic (fcc), on the other hand the second identification

was achieved with the pdf card for Fe3O4 (magnetite) 04-016-4197. The crystallize size of the

second material was obtained by considering Iron oxide Fe3O4 diffraction peak (2θ= 34.75˚),

corresponding to Miller indices (230). This is revealed that the resultant particles were

orthorhombic [18].

Fig. 2. XRD diffractogram of nano-magnetite-trevorite.

3.2 Morphology, topography, structure and phases

At a first glance SEM images recorded on the synthesis products show particles with

irregular shapes (see Fig. 3). A detailed analysis at downscale reveals spongy-like structures

consisting of nanoparticles form the spongy structure. Table 1 shows the composition analysis of

the nano-magnetite-trevorite compound powder obtained by EDX test. The EDX results confirm

the percentage ratio of Fe, Ni, and O, in the synthesized composite.

Fig. 3. SEM image of magnetite-trevorite nanoparticles.

Table 2. EDX analysis of magnetite-trevorite nanoparticles.

Element Weight (%) Atomic (%) Error (%)

O K 11.91 32.15 6.35

Fe K 81.13 62.73 1.68

Ni K 6.96 5.12 3.71

3.3 Thermal analysis

Thermal analysis of pure paraffin and paraffin/ nano-magnetite-trevorite compound was

tested at various loadings of nano-magnetite-trevorite particles 0wt%, 0.5wt%, 1wt%, and 2wt%

with 10oC/min heating and cooling rate (see Fig. 4). The phase transition for both, solid-solid

phase occurring at lower temperature and solid-liquid phase at higher temperature were

distinguished. The magnetite/trevorite particles impact on solidification and melting temperatures

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of the composite are exposed in Table 3. The table readings show a slight change in the results of

solidification and melting temperatures under the effect of nano-magnetite-trevorite particles.

Solid-solid transition occur at 35.46oC, followed by solid–liquid transition at 52.34

oC. Tt is the

phase change temperatures measured during heating and cooling were denoted as the transition

temperature, Tm is the melting temperature and Tc is the crystallization temperature, respectively.

The first peak appear at solid-solid transition can be interpreted a monoclinic crystal disordering to

a pseudo-hexagonal crystal before isotropization of the melt which is a paraffin property [19].

Fig. 4. (left) DSC thermograms of paraffin with different loadings of magnetite/trevorite particles (^exo);

(right) SHC heat storage in paraffin- magnetite/trevorite particles composites for different wt.% loadings.

(FeNi-0, FeNi -0.5, FeNi -1, and FeNi -2,) with the heating and cooling rate of 10oC/min.

Table 3 indicates the enthalpy of pure paraffin and various ratios of paraffin/nano-

magnetite-trevorite composite. The enthalpy of the samples was estimated by integrating the area

under the DSC curve. Fig. 4 (left) indicates the storage heat capacity (SHC) of pure paraffin and

paraffin/nano-magnetite-trevorite particles. Results shows that SHC increased for lower

concentration and decreased for higher nanoparticle loadings. Latent heat is a crucial factor for

PCMs, since it determines the thermal capacity. Latent heat improvement detected at 0.5wt%

loading of nano-magnetite-trevorite particles can be attributed to the high surface area of the

nanoparticle which leads to an intermolecular interaction with the surrounding medium. Another

factor improving the surface interaction paraffin/ nano-magnetite-trevorite compound is the

different coordination geometry of nano-magnetite-trevorite particles structure related to surface

defect sites [20]. Previous researches have evidenced also enhancement of latent heat when adding

different nano-materials, such as carbon nanotubes and nano-SiO2 to PCM [21-22].

Table 3. The transition temperature Tt, melting temperature Tm, crystallization temperature Tc

and latent heat of the pure paraffin and the mixtures

Sample %wt FeNi Tt (oC) Tm (

oC) Tc (

oC) ∆𝐇 (J/g)

FeNi-0% 0.00 35.46 52.34 60.46 112.99

FeNi -0.5% 0.50 35.90 52.29 60.79 116.22

FeNi -1% 1.00 35.68 52.35 60.59 112.51

FeNi -2% 2.00 36.14 52.40 60.35 98.44

Table 4. Specific heat capacities of the solid and liquid phase at (s, 15 oC) and (l, 80

oC).

Sample FeNi -0% FeNi-0.5% FeNi -1% FeNi -2%

Cp (s, 15 oC)(J/g.K) 1.569 1.669 1.555 1.434

Cp (l, 80 oC) (J/g.K) 1.932 2.047 1.933 1.797

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The activation energy of the phase transitions is estimated by Kissinger method, using

DSC data of heating the paraffin based samples at different heating rates and plotting ln (V/Tp2) as

function of 1000/Tp [23] (Fig. 5), where V is the heating rate (oC/s) and Tp the peak temperature

(K). The activation energy (Ea) is calculated from the slope of the linear fit of the graph and is

defined as the least required energy for attaining the specific phase transition.

Fig. 5. Graph ln (V/Tp2) vs. 1000/Tp. - Kissinger method for the

paraffin- magnetite-trevorite mixtures.

Table 5 expose the activation energy, pre-exponential factor and correlation coefficient.

The results shows a slight change of activation energy for both solid-solid and solid-liquid phase

transitions with various nanoparticles loadings. One can note an increase of the activation energy

value for the solid-liquid transitions in 0.5-1 wt.% nanoparticle loading. This increasing tendency

is correlated with thermal analysis results that the composite material has a higher latent heat. The

nano-magnetite-trevorite dispersed into the organic lamellar phase may change the dynamics of

the melting process since the nanoparticles acted as a thermal hinder in the paraffin matrix.

Table 4. The activation energy, pre-exponential factor, and correlation coefficient.

Solid-Solid Solid-Liquid

Sample %wt.

FeNi

Ea

(kJ/mol)

ln A R Ea

(kJ/mol)

ln A R

FeNi -0% 0.0 480±36 184.46 0.98 398±72 145.10 0.91

FeNi -0.5% 0.5 504±38 193.49 0.98 495±48 180.11 0.96

FeNi -1% 1.0 421±33 161.84 0.97 594±83 284.16 0.94

FeNi -2% 2.0 384±35 147.48 0.97 413±53 149.83 0.94

3.4 Heat transfer rates Heat transfer rates for the nano-magnetite-trevorite-enhanced paraffin have been

investigated using the experimental test rig that provides constant heat flux from one side of the

enclosure while the rest is thermally insulated. The required time to achieve melting is recorded

for various loadings of particles by studying the evolution of the temperature values at different

locations inside the cell (Fig. 6). The recorded results present a decrease in the charging time with

increasing loadings compared to pure paraffin: 28.0%, 30%, and 34% for the 0.5wt.%, 1wt.%, and

2wt.% loadings respectively (Table 5). However, results show an increase in heat transfer rates in

the range 0.5 to 1wt.%, while in the range 1 to 2wt.% the transfer rates show a slight increase. This

can be explained as a decrease in the impact of natural convection in the melt fraction due to an

increase in dynamic viscosity, or as agglomeration and precipitation occurring at high

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nanoparticles loadings. In some cases, the effects of high viscosity may exceed the enhancement in

heat transfer rates, as observed both experimentally and numerically in past research [24-25].

Table 5. Time duration of a single melt cycle, as recorded by TC6.

Sample FeNi Loading

[wt.%]

Melting point Tm

[°C]

Time until melting

[s]

FeNi -0% 0.0 55.00 9183

FeNi -0.5% 0.5 55.01 6595

FeNi -1% 1.0 55.01 6426

FeNi -2% 2.0 55.02 6023

Readings obtained by the attached thermocouples show that the temperatures in the upper

part of the test cell (TC3, TC5, and TC7) are generally higher than the lower part due to the strong

natural convection currents, which begin within the melted paraffin next to the active heated wall.

The direction of the convection currents is clockwise in the direction of the solid-liquid interface.

When these currents reach the interface, thermal energy is transferred from the liquid phase toward

the solid phase, and the resident liquid phase becomes colder. The density of the cold phase is

lower than the hot phase, which forces the liquid to descend along the interface from the upper part

toward the lower part of the test rig, as recorded by TC2, TC4, and TC6 (high temperature

difference between the temperature readings on the upper and lower part of the test rig). On the

lower part of the cell, diffusive heat transfer is dominant, while on the upper part convective heat

transfer takes over. The velocity of the melting front phase increases with raising the loading of the

nano-magnetite-trevorite particles (Figure 6), which lead in minimizing the charging time of the

composite. One can note an improvement in the meting front phase velocity with addition of the

nanoparticles and the maximum reading recorded at 2wt% of the nanoparticles ratio.

Fig. 6. Paraffin, FeNi-0.5wt, 1wt%, and 2 wt.%: Temperature evolution for the seven

thermocouples inside the test rig and the velocity of the melting front phase of paraffin ІІ with different

values of the Fe-Ni particles additives.

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Fig. 7. Latent heat and thermal conductivity of the paraffin/FeNi as a function of mass fraction (left);

velocity of the melting front phase with different nanoparticles loadings(right).

4. Conclusions In summary, we have demonstrated that magnetite-trevorite composite with low loadings

have an effective role in thermal energy storage applications. Slight enhancement in latent heat

was detected with specific loading of nanoparticles 0.5wt%, while higher loadings did not

determine an increase compared to pure paraffin. The same behavior was found in SHC.

Improvement in latent heat and SHC can be explained by the high surface area to the volume, and

by the surface defect sites of nano-magnetite-trevorite compound introduced by different geometry

conditions. The fact of keeping the latent heat and specific heat capacity at the same level after the

nanoparticles addition make it suitable to be tested for heat transfer improvement of paraffin.

Results of the effect of nanomagnetite-trevorite addition on the thermal properties prove an

increase in the melting front phase velocity due to the charging time decrease with increasing the

nanoparticle content. However the percent of improvement of the melting front phase velocity was

reduced with the higher concentration which can be interpreted as a result for agglomeration or

increasing the viscosity of the used mixture.

Acknowledgements

This work was supported by the Ministry of Science and Technology in Baghdad and by

the Romanian Space Agency, Star Program, project no. 134/2017.

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